It is known, e.g., from Patent Specification Nos. U.S. Pat. No. 4,162,781; GB-A-2056660 and GB-A-1571245 to read optical markings on a tape rule and derive therefrom information about the position of the tape. If only movement and direction are required to be known, then only two photosensors are required. A workable tape system can be made using this approach and an implementation of it is described in U.S. Pat. No. -4,161,781. In essence, such an arrangement counts output state transitions as the tape moves from a known position to an unknown position whose displacement is to be measured. But positional accuracy relies on accurate and reliable knowledge of the first position and on accurate transition counting. Should any error occur in counting during a movement, as a result of damage to the line of marks being counted, then subsequently the known position will be in error, and the tape as a whole will be useless.
A method and apparatus for determining the absolute position of a movable element mounted for movement along a path is described in U.S. Pat. No. 4,009,377 (Elms) and is based on the development of a pseudo-randum sequence of indicia that uniquely define the position of the movable element on the path. But the provision of a separate timing track is essential and errors in reading the timing track would result in spurious detected indicia and large errors in measured position from which recovery would be difficult. The use of binary sequences to address sections of a rotating device is described by B. Arazi, Electronics Letters, 20, 61-62 (19 Jan. 1984).